In this study, the analysis of marrow megakaryocytes (using fluorescence-activated flow cytometry) will be combined with platelet flow cytometric and platelet kinetic measurements to characterize platelet production and destruction in patients with megakaryocytopoiesis. We will test the hypothesis that alteration in megakaryocytic ploidy constitutes important compensatory mechanisms responding to changes in peripheral platelet demand, and that the resultant physiologic changes in ploidy and size are subject to pathologic aberrations.